Imaging systems

ABSTRACT

An electrostatographic imaging system of the liquid development type with the space between the development electrode and the imaging surface being maintained substantially constant during development by the repulsive forces of magnetism.

I United States Patent 1 1111 3,836,384 Tamai Sept. 17, 1974 [5 IMAGING SYSTEMS 3,196,566 7 1965 Littlefield 335 306 x 1 1 Inventor: Yam Tami, Asaka, Japan 15181323 111322 3211;131:113..- 33311131111111: 115/1351 73 Assigneez Fuji m Film Company, Ltd 3,322,048 5/1967 Fauser et a1. 1 18/637 X I Tokyo Japan 3,365,324 H1968 Blake 117/37 3,416,494 12/1968 Hudson 118/637 [22] Filed: Sept. 26, 1969 3,434,084 3/1969 Milligan 308/10 x 3,441,331 4/1969 Kesling 308/10 X (Under Rule 47) 3,512,852 5/1970 North 308/10 3,557,751 1/1971 Kushima 118/637 [21] Appl' 20265 3,560,203 2 1971 Honjo el al. 117 37 x [30] Foreign Application Priority Data 061. 1, 1968 Japan 43-71385 prlmay Sofocleous [52] US. Cl 117/37 LE, 117/934 A, 118/637,

' 335/219 57 ABSTRACT [51] Int. Cl. G03g 13/10, 603g 15/10 of Search .Q.... LE, An electrostatographic imaging ystem of the 8/637, 1316- 23; 308/101 335/219, 296, development type with the space between the devel- 306; 52/DIG- 4 opment electrode and the imaging surface being maintained substantially constant during development by 1 References Clted the repulsive forces of magnetism.

UNITED STATES PATENTS 3,029,824 4/1962 Goodell 308/10 X 21 Claims, 3 Drawing Figures NNNNNNNN SSS-SSSSS PATENIEU I 3.836.384

FIG.

NNNNNNNN S S S S S S S 8 FIG. 2

INVENTOR. YASHIO TAMAI ATTORNEY IMAGING SYSTEMS BACKGROUND OF THE INVENTION This invention relates to imaging systems, and more particularly, to improved liquid development systems free from edge effects during development.

The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic xerographic process as taught by C. F. Carlson in US. Pat. No. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as toner. The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image'may subsequently be permanently affixed to a support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and. then exposing the layer to a light shadow image, one may form the latent image directly by charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.

Similar methods are known for applying the electroscopic particles to the electrostatic image to be developed. Included with this group are the cascade development technique disclosed by E. N. Wise in US. Pat. No. 2,618,552; the powder cloud technique disclosed by C. F. Carlson in US. Pat. No. 2,221,776 and the magnetic brush process disclosed, for example, in U.S. Pat. No. 2,874,063.

Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern, the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electrophoretic development of an electrostatic latent image may, for example, be obtained by pouring the developer over the image bearing surface, by imersing the imaging surface in a pool of the developer or by presenting the liquid developer on a smooth surface roller and moving the roller against the imaging surface. The liquid development technique has been shown to provide picture images of excellent quality and to provide particular advantages over other development methods in offering ease in handling and high development speed.

The advantages of liquid development techniques have been embodied in commercial development modes of varying configuration. In a particularly satisfactory commercial embodiment a sensitive paper prepared by blending a photoconductive powder with an insulating resin binder on a surface of an electroconductive base is uniformly charged in the dark and exposed to a light-and-shadow image to be reproduced. The electrostatic latent image is developed with a liquid developer having finely divided particles suspended in an insulating liquid. The fine particles have a specific electric charge determined by the nature and surface of the particles and their dispersion within the insulating liquid. The charged particles are usually colored and are attracted in response to the electrostatic force field pattern of the image to the sensitive paper to provide a positive image. A negative image may be obtained through repulsion development by changing the relationship between the polarity of electric charge on the sensitive paper and the polarity of the charged parti cles.

The density of developer particles deposited on the imaging surface is dependent on the concentration of the developer particles in the insulating liquid and on the charge of the electrostatic image. Where the electric charge is uniformly distributed throughout a wide area the electric field becomes stronger at the peripheral portion and weaker at the central portion. Moreover, the surface electric charge undergoes darkness attenuation within the liquid. developer. Thus the central portion of such a charge pattern exhibits negligible attraction for the developer particles and consequently the peripheral portion of the image area is developed darkly and the central portion less darkly. This edge effect in the development of an electrostatic latent image may be eliminated by the use of a development electrode whose function it is to draw the lines of force of the electrostatic latent image externally above the image bearing surface. By positioning the development electrode close to the surface of the imaging surface improved solid area coverage is obtained by increasing the amount of the lines of force that are external to the image bearing surface and hence available to attract electrostatically charged marking particles.

While capable of producing statisfactory images these liquid development systems in general suffer deficiencies in certain areas and are in need of further development and improvement. The effect of the development electrode in reducing the edge effect is found to increase with decreasing distance between the developing electrode and the imaging surface for the images to be developed. Since the liquid developer must pass between the development electrode and the image bearing surface if the space between the image bearing surface and development electrode is excessively reduced there will be an insufficient supply of the liquid developer to the imaging surface to develop the electrostatic latent image. Consequently the development will be insufficient and may give rise to the edge effect or require an extension of the developing time. On the other hand, if the surface being developed comes into direct contact with the developing electrode the fine particle image already formed on the surface being developed may be destroyed. Maintaining a fixed space constantly between the development electrode and imaging surface therefore becomes quite important. in actual practice however, it has been found to be quite difficult to maintain this space between the imaging space and the development electrode. It is therefore clear that there is a continuing need for a better liquid development system employing a development electrode.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a developing system which overcomes the above noted deficiencies.

It is another object of this invention to provide a liquid development system providing improved solid area coverage.

It is another object of this invention to provide a development system capable of increased development speed.

' It is another object of this invention to provide means for maintaining the space between the development electrode and the imaging surface constant.

It is another object'of this invention to provide a liquid development system which produces developed images with substantiallyno edge effect.

It is another object of this invention to provide novel development electrode. v

It is another object of this invention to provide a development electrode of ,electroconductive material.

It is another object of this invention to provide a liquid development system superior to known systems.

The above objects and others are accomplished generally speaking by providing an electrostatographic imaging system of the liquid development type wherein the space between the development electrode and the imaging surface remains substantially constant and development is accomplished such that the developer particles on the imaging surface in image configuration do not come into direct mechanical contact with the development electrode. More specifically the repulsive forces of magnetism are employed for maintaining the space constant between the development electrode and the'imaging surface. The imaging surface is positioned in the space which is created by the magnetic repulsion generated by a magnet disposed on the base side of the imaging surface and a development electrode made partially or wholly of a magnet. The developer liquid is supplied to the space created by the development electrode and the imaging surface to provide development of the electrostatic latent image. The dimensions of the space between the imaging surface and the development electrode are determined by the magnetic repulsive force and the weight of the development electrode and therefore can be varied freely by controlling the weight or'the magnetic forces.

The invention may be further illustrated by reference to the accompanying drawings in which:

FIG. 1 is a longitudinal sectional side elevation of a device according to the present invention.

FIG. 2 is also a longitudinal sectional side elevation of an alternative embodiment according to the present invention. I

FIG. 3 is an enlarged longitudinal sectional side elevation of a development electrode.

In the developing system shown in FIG. 1, 11 denotes a receptacle containing liquid developer 12. On the bottom surface thereof there is placed an image bearing surface which in this embodiment is the sensitive paper discussed above having the sensitive layer 14 formed on the base 13, with the surface bearing the electrostatic latent image facing upwardly. The numeral l denotes the development electrode here comprising a magnet. In this drawing the development electrode 15 is positioned such that the N pole is in the lower side and the S pole on the upper side. To facilitate handling a handle 16 is provided on the upper side of the development electrode. The bottom surface of the developer receptacle 11 is made of a magnet 17, which is so positioned that the polarity of the same sign as that of the lower surface of the developing electrode is always formed on the upper side thereof. The space between the lower side of the development electrode 15 and the upper surface of the magnet 17 is determined by the balance of the weight of the development electrode 15 and the magnetic repulsive force generated by the two confronting magnets 15 and 17. The development electrode 15 serves as the nearby electrode against the imaging surface and the lower surface of the development electrode is therefore preferably smooth and flat. The imaging surface is electrically charged uniformly in the dark, exposed to a light and shadow image, and placed at the bottom surface of the receptacle 11 filled with the liquid developer 12 in such a way that it will not float up. The development electrode 15 which is positioned and sunk from above into the liquid developer in the developer receptacle is not permitted to fall to the extent of coming into contact with the surface of the sensitive paper but rather is suspended at a distance from the sensitive paper where the magnetic repulsive force and the weight of the development electrode are balanced. It the supply of the liquid developer to this is insufficient, the situation can be improved by occasionally lifting or parting the development electrode 15 upwardly. The size of ,the space is constant unless an external force is applied to the development electrode 15. When development is completed the development electrode 15 is removed and the sensitive paper with the developed image on the surface is taken out. p 7

While the embodiment in FIG. 1 employs the use of permanent magnets, electromagnets magnetized with DC electricity may also be used. The embodiment illustrated in FIG. 2 provides such use. At the bottom of the developer receptacle 11 there is installed an electromagnet composed of an iron core 21 and the coil 22.

As in the embodiment in FIG. 1 the electromagnet is designed such that the lower surface of the development electrode 15 and the upper surface of the electromagnet 21 assume the same polarity. Where an electromagnet is employed, the space between the developing electrode 15 and the surface of the sensitive paper being developed can be determined by the intensity of the magnetization of the electromagnet. To accelerate the supply of the liquid developer, the developing electrode 15 may be moved in a vertical direction by controlling the electric current flowing to the coil 22, thereby varying the intensity of magnetization. While both FIGS. 1 and 2 depict embodiments in which the two magnets confront each other with the N poles on the inside and opposing each other, it is permissible to place the magnets in such a position that the S poles face each other.

Where the supply of the liquid developer is accelerated by moving the development electrode 15 in the vertical direction in the embodiment of either FIG. 1 or FIG. 2, it is advisable to provide small orifices 31 in the longitudinal direction in the development electrode 15 as illustrated in FIG. 3. As the development electrode 15 moves upward and downward, liquid developer is passed through the orifices 31 and supplied to the image bearing surface.

Development of an electrostatic latent image according to the technique described herein may be obtained on any suitable conventional electrostatographic imaging surface. Essentially, any surface upon which an electrostatic charge pattern amy be formed or developed may be employed. Typical imaging surfaces include the sensitive paper discussed above which may be a flexible sheet of paper with a zinc oxide binder coating. In addition, a flat plate of selenium deposited on an aluminum base through vacuum evaporation provides an effectiveimaging surface.

Development of an electrostatic latent image according to the described technique may be obtained with any suitable conventional liquid developer. Typical liquid developer contain electroscopic marking particles dispersed in an insulating liquid vehicle and may also contain control agents and suspending agents for their well-known functions. The liquid employed must have a relatively high insulating value to avoid discharge of the electrostatic latent image, Typical conventional materials include hydrocarbons such as benzene, xy-

lene, hexane, naphtha, kerosene; halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene and chloroform. Typical conventional electroscopic particles include charcoal, magnesium oxide, lithopone, cadmium yellow, chrome yellow, cobalt blue, cadmium red, burnt sienna, Hansa yellow, rose bengal, phthalocyanine. In the practice of the development technique herein described, it is preferred to provide a liquid developer containing electroscopic particles which are non-magnetic or free from magnetism to thereby prevent them from being attracted by the magnetic poles in the development zone. The electroscopic particles are conventionally dispersed or suspended in the liquid by stirring or agitation, and where a highly uniform and stable suspension is desired, the suspension may be passed through a colloid mill.

Instead of shorting the development electrode to the imaging surface, either positive or negative, potentials can be applied to the electrode or sufficient bias potential may be applied to obtain reversal development in conventional manner.

For any particular development system, the spacing of the development electrode from the imaging surface may be readily determined by one skilled in the art. conventionally, the development electrode is normally spaced within about one-eighth inch of the imaging surface. Typical conventional electrode distances in electrophoretic development are in the range of from about 25 to 100 microns.

The development electrode may be of any suitable material. Typically, the development electrode is a ferromagnetic article having electroconductivity. Where a ferrite or similar substance is employed, it is sufficient to have an electroconductive ferromagnetic or nonmagnetic article held fast on its surface.

FIG. 1 and FIG. 2 illustrate examples in which solid magnets are employed. Where separate magnets of the same polarity with small orifices through them are employed as shown in FIG. 3, the magnetic repulsive force increases with decreasing distance between the magnet installed behind confronting sensitive paper and the magnet serving as a developement electrode. Consequently, control of the space is easy. When either of the development electrodes shown in FIGS. 1 and 2 is used and the space between the development electrode and imaging surface becomes smaller than a development distance, the proportional relationship between the magnetic repulsive force and distance is terminated.

According to the development method of this invention the repulsive force exerted between the magnet provided behind the image bearing surface and the magnet disposed within the developing electrode or the magnetized developing electrode is balanced, as described in detail above, with the weight of the development electrode so that a very small space can be maintained adjustably and stably between the surface of the image bearing surface being developed and the development electrode. Consequently, a continuous development process producing an essentially flawless picture image of excellent quality substantially free from the edge effect is obtained.

Although particular embodiments have been set forth using the development system and technique of this invention, these are merely intended as illustrations of the present invention. There are other systems and techniques which may be substituted for those described. Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure which modifications are intended to be included within the scope of this invention. For example, the magnetic repulsive force exerted between the magnet provided behind the image bearing surface and the development electrode may be employed to maintain and adjust the space between the imaging surface and the development electrode in response to forces other than the weight of the development electrode.

What is claimed is:

1. The method of developing an electrostatic latentimage comprising providing an electrostatographic imaging member bearing an electrostatic latent image on an imaging surface, placing a development electrode which is at least partially magnetic at a position adjacent to but spaced from and confonting said imaging surface, juxtapositioning a magnet on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said developement electrode and said magnet to suspend said development electrode and maintain a substantially constant space between said developement electrode and said imaging surface, and supplying liquid developer to said space between said development electrode and said imaging surface.

2. The method of claim 1 wherein the magnetic repulsive force between said development electrode and said magnet is sufficient to balance the weight of said development electrode.

3. The method of developing an electrostatic latent image comprising providing an electrostatographic imaging member bearing an electrostatic latent image on an imaging surface, and contacting said imaging surface with liquid developer in a development zone comprising a magnetizable development electrode positioned adjacent to but spaced from and confronting said imaging surface and a magnet juxtapositioned on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to suspend said development electrode and maintain a substantially constant space between said development electrode and said imaging surface.

4. The method of claim 3 wherein said development electrode is a magnet.

5. The method of claim 3 wherein said development electrode is provided with a plurality of small orifices.

6. The method of claim 3 including increasing said magnetic repulsive force between said development electrode and said magnet to move said development electrode in a direction perpendicular to the plane of said imaging surface so as to accelerate the supply of said liquid developer to said development zone.

7. The method of claim 3 wherein said imaging surface comprises a flexible sheet of paper with a zinc oxide binder coating.

8. The method of claim 3 wherein said development electrode comprises an electroconductive material.

9. The method of claim 3 wherein said magnet is a permanent magnet.

10. The method of claim 3 wherein said magnet is an electromagnet.

11. The method of claim 3 wherein said liquid developer comprises nonmagnetic electroscopic particles dispersed in an insulating liquid.

12. The method of claim 3 wherein said space between said development electrode and said imaging surface is maintained substantially uniformly constant during development of said electrostatic latent image.

13. The method of developing an electrostatic latent image comprising providing an electrostatographic imaging member bearing an electrostatic latent image on an imaging surface, and contacting said imaging surface with liquid developer in a development zone comprising a magnetizable development electrode positioned-adjacent to but spaced from and confronting said imaging surface and a magnet juxtapositioned on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to balance the weight of said development electrode and maintain a substantially constant space between said development electrode and said imaging surface.

14. The method of claim 13 including adding additional weight to said development electrode to reduce the space between said development electrode and said imaging surface.

15. An imaging apparatus comprising an electrostatographic imaging member having an imaging surface, a development electrode which is at least partially magnetic positioned adjacent to but spaced from and confronting said imaging surface, a magnet juxtapositioned on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to suspend said development electrode and maintain a substantially constant space between said development electrode and said imaging surface, and means to supply liquid developer to said space between said development electrode and said imaging surface.

16. An imaging apparatus according to claim 15 wherein the magnetic repulsive force between said development electrode and said magnet may be increased to move said development electrode in a direction perpendicular to the plane of said imaging surface.

17. An imaging apparatus according to claim 15 wherein said development electrode comprises an electroconductive material.

18. An imaging apparatus according to claim 15 wherein said development electrode comprises a magnet.

19. An imaging apparatus according to claim 15 wherein said magnet comprises a permanent magnet.

20. An imaging apparatus according to claim 15 wherein said magnet comprises an electromagnet.

21. An imaging apparatus comprising a receptacle containing liquid developer, means to position within said receptacle an electrostatographic imaging member having an imaging surface, a development electrode which is at least partially magnetic positioned adjacent to but spaced from and confronting said imaging surface, a magnet juxtapositioned on the opposite side of said imaging member away'from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to suspend said development electrode and maintain a substantially constant space between said development electrode and said imaging surface, and means to supply liquid developer to said space between said development electrode and said imaging surface. 

2. The method of claim 1 wherein the magnetic repulsive force between said development electrode and said magnet is sufficient to balance the weight of said development electrode.
 3. The method of developing an electrostatic latent image comprising providing an electrostatographic imaging member bearing an electrostatic latent image on an imaging surface, and contacting said imaging surface with liquid developer in a development zone comprising a magnetizable development electrode positioned adjacent to but spaced from and confronting said imaging surface and a magnet juxtapositioned on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to suspend said development electrode and maintain a substantially constant space between said development electrode and said imaging surface.
 4. The method of claim 3 wherein said development electrode is a magnet.
 5. The method of claim 3 wherein said development electrode is provided with a plurality of small orifices.
 6. The method of claim 3 including increasing said magnetic repulsive force between said development electrode and said magnet to move said development electrode in a direction perpendicular to the plane of said imaging surface so as to accelerate the supply of said liquid developer to said development zone.
 7. The method of claim 3 wherein said imaging surface comprises a flexible sheet of paper with a zinc oxide binder coating.
 8. The method of claim 3 wherein said development electrode comprises an electroconductive material.
 9. The method of claim 3 wherein said magnet is a permanent magnet.
 10. The method of claim 3 wherein said magnet is an electromagnet.
 11. The method of claim 3 wherein said liquid developer comprises nonmagnetic electroscopic particles dispersed in an insulating liquid.
 12. The method of claim 3 wherein said space between said development electrode and said imaging surface is maintained substantially uniformly constant during development of said electrostatic latent image.
 13. The method of developing an electrostatic latent image comprising providing an electrostatographic imaging member bearing an electrostatic latent image on an imaging surface, and contacting said imaging surface with liquid developer in a development zone comprising a magnetizable development electrode positioned adjacent to but spaced from and confronting said imaging surface and a magnet juxtapositioned on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to balance the weight of said development electrode and maintain a substantially constant space between said development electrode and said imaging surface.
 14. The method of claim 13 including adding additional weight to said development electrode to reduce the space between said development electrode and said imaging surface.
 15. An imaging apparatus comprising an electrostatographic imaging member having an imaging surface, a development electrode which is at least partially magnetic positioned adjacent to but spaced from and confronting said imaging surface, a magnet juXtapositioned on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to suspend said development electrode and maintain a substantially constant space between said development electrode and said imaging surface, and means to supply liquid developer to said space between said development electrode and said imaging surface.
 16. An imaging apparatus according to claim 15 wherein the magnetic repulsive force between said development electrode and said magnet may be increased to move said development electrode in a direction perpendicular to the plane of said imaging surface.
 17. An imaging apparatus according to claim 15 wherein said development electrode comprises an electroconductive material.
 18. An imaging apparatus according to claim 15 wherein said development electrode comprises a magnet.
 19. An imaging apparatus according to claim 15 wherein said magnet comprises a permanent magnet.
 20. An imaging apparatus according to claim 15 wherein said magnet comprises an electromagnet.
 21. An imaging apparatus comprising a receptacle containing liquid developer, means to position within said receptacle an electrostatographic imaging member having an imaging surface, a development electrode which is at least partially magnetic positioned adjacent to but spaced from and confronting said imaging surface, a magnet juxtapositioned on the opposite side of said imaging member away from said imaging surface to provide sufficient magnetic repulsive force between said development electrode and said magnet to suspend said development electrode and maintain a substantially constant space between said development electrode and said imaging surface, and means to supply liquid developer to said space between said development electrode and said imaging surface. 